Lymphotoxin (LT;TNF-beta) contributes to defense against tumors and virus-infected cells on the one hand and to disease pathogenesis on the other. It may also play a role in T and B cell development in its ability to induce apoptosis and proliferation. Our long range goals are to define the molecular basis of LT regulation and to understand how this impacts on the protein's biologic significance. The focus is on LT, but its special relationship with the functionally similar and genetically linked TNF-alpha will also be considered. Defined genetic elements that regulate LT expression in T and B cell lines are transcriptional start sites, minimal promoters, and upstream negative and positive regulatory elements. These include a NF-kappa-B site important for constitutive LT production in T cells and a poly (dA-dT) rich HMG-1 binding sequence that contributes to LT promoter activity in pre B cells. LT's tight linkage to the TNF-alpha gene, and the fact that in many cells the two cytokines are activated by the same signals, gives rise to the hypothesis that the region between the genes (IGR) contains shared regulatory elements that may influence expression of both within the context of their promoters. The Specific Aims are: 1. To evaluate the role in LT regulation of sequences shared between the LT and TNF-alpha genes. 2. To evaluate the role of HMG-1 in LT regulation. 3. To identify additional regulatory elements within and outside-the LT gene. 4. To evaluate the biologic role of LT and LT and TNF-alpha particularly in the development of T and B cell competence. DNA protein binding studies, functional analyses by transient transfections and in transgenic mice, and homologous recombination and gene knock-out techniques will be used. These experiments will contribute to our understanding of LT's biological significance and enhance our ability to harness the expression of this potent cytokine in clinical situations. Experiments that test the consequences of elimination of LT expression will provide information concerning its biological role and its redundancy with TNF-alpha.